1. Field of the Invention
The present invention relates to, for example, an electric-powered linear actuator used in an operating unit of a medical/nursing-care bed, and particularly to the functioning structure of a micro switch for limiting the operating range of the actuator.
2. Related Art Statement
In the medical and nursing-care sectors, an electric-powered bed which moves a bed upward or downward, reclines, and carries out other operations by electric-powered actuators has been widely used to reduce burden on patients and caretakers. For the electric-powered bed actuators, a linear actuator of a feed-screw type is used in many cases. The feed-screw type linear actuator is equipped with a shaft having a male screw section and a female screw member (feed nut) that is threaded onto the shaft. As the shaft is rotated by an electric motor, the female screw member moves along the shaft.
The feed-screw type linear actuator is equipped with a worm wheel speed reduction device that reduces the speed of the rotation of the motor before conveying the rotation of the motor to the shaft. A movement cylinder is connected to the female screw member. The movement cylinder moves together with the female screw member. A link mechanism is connected to the movement cylinder through a connector: The link mechanism is designed to move a bed upward or downward as well as to tilt the back or knee portion of the bed. As the motor rotates, the shaft slows down and rotates. As the shaft rotates, the movement cylinder moves. When the movement cylinder moves forward or backward relative to a case of the actuator, the bed moves upward or downward or reclines by means of the link mechanism connected to the movement cylinder.
Meanwhile, a limit switch (micro switch) is used in the above kind of linear actuators to detect the end of strokes of the movement cylinder. For example, in the case of Patent Document 1, the end of strokes of the movement cylinder is detected by a limit switch. After the movement cylinder comes to the end of the strokes, the motor automatically stops operating.
What is also proposed is a linear actuator equipped with a striker that is used to operate a limit switch in order to secure the stop position accuracy of an actuator (Patent Document 2, for example). FIG. 5 is an explanatory diagram showing the configuration of a linear actuator equipped with a striker. A linear actuator 50 of FIG. 5 includes a drive-source motor 51, a motor unit 53 that houses a speed reduction mechanism unit 52, and a limit switch unit 55 where limit switches 54 are disposed. The limit switches 54a and 54b are disposed at the upper- and lower-limit positions of actuator strokes in the limit switch unit 55.
In the limit switch unit 55, a long and thin base plate 56 is so provided as to correspond to the strokes. On the base plate 56, the limit switches 54a and 54b are disposed for upper and lower limits. The limit switches 54a and 54b are put into units along with other circuit elements and are disposed around a piston (movement cylinder) 58 in a case 57. The piston 58 is mounted firmly on a screw nut 60 that is threaded onto a shaft 59. The shaft 59 is driven and rotated by the motor 51. As the shaft 59 rotates, the piston 58 moves together with the screw nut 60 in the axial direction. A striker 62 is attached to the screw nut 60 through a spring 61. The striker 62 comes in contact with the limit switches 54a and 54b at a movement limit position of the linear actuator 50. After the striker 62 pushes the buttons of the limit switches 54a and 54b, the motor 51 stops. The actuator, therefore, stops at a predetermined movement limit.
However, such a linear actuator requires various limit switch units because of differences in the specifications (length of strokes) of actuators. Accordingly, a base plate and other components need to be prepared as new components for each set of specifications. As a result, the problem is that the number of types of components increases, resulting in increases in production costs. Moreover, the base plate attached to the limit switch is narrow, long and thin and therefore is easy to deform. The deformation of the base plate could be a factor in decreasing the reliability of the actuator. Furthermore, it is difficult to carry out soldering work on the long and thin base plate. It is also difficult to make the long and thin base plate with a die, which is a factor in increasing costs. Moreover, a holder is required to keep the long base plate inside the case. Therefore, the number of components increases. Furthermore, since it is necessary to put the base plate, limit switches, strikers and other components in the case, the portion that houses the above components projects from the device. Therefore, another problem is that the device becomes large in size and is not excellent in terms of layout.
Meanwhile, in a device such as the one shown in FIG. 5, the operation of attaching the striker 62 to the screw nut 60 involves the operation of putting the spring 61 between both. The workability is not good and the structure of the device becomes complex. Furthermore, as the striker 62 moves in the axial direction, the striker 62 pushes the buttons of the limit switches 54a and 54b up, then, it is impossible to adopt an operation method recommended by switch manufacturers.